1. Field of the Invention
This invention relates to hydrogen absorption systems that utilize the heats of absorption as heat sources for heat pump systems and the heats of desorption as heat sinks for refrigeration systems.
2. Brief Description of the Prior Art
In U.S. Pat. No. 3,504,494, a closed cycle method for intermittently producing high energy steam has been described in which the system consists of a power cycle followed by a recharging cycle. In the power cycle, a first hydride bed is heated to desorb hydrogen gas therefrom. The gas flows to a second hydride bed where, the hydrogen can be absorbed at a lower temperature than the temperature of the desorption from the first bed. Absorption of the hydrogen by the second bed releases the heat of absorption which is used to convert water to steam. The steam is used for power production, and the residual heat remaining in the steam after such power production is used for heating the first hydride bed and enhancing the desorption of hydrogen therefrom. After complete desorption of the hydrogen from the first bed and condensation of the residual steam, the recharging cycle is started. In the recharging cycle, the second hydride bed is heated by a heat source which can be a low energy isotope source, a chemical heater, an electrical heater or other suitable source of thermal energy. The second bed is thus caused to dehydride, and the first bed is cooled so that it can absorb the hydrogen desorbed from the second bed preparatory to recommencing the power cycle after recharging.
U.S. Pat. No. 3,943,719 describes hydride-dehydride-hydrogen (HDH) cycles used for the production of simultaneous and continuous power and refrigeration by means of thermochemical compression utilizing hydriding materials. For continuously supplying relatively high pressure hydrogen gas, a plurality of hydride-dehydride reactors are provided and are operated in out-of-phase or staggered sequence so that during the period when low-pressure, relatively cool hydrogen gas is being charged to one of the reactors, another is being activated and another being dehydrided to produce high pressure hydrogen gas. The pressure energy of the gas thus developed in the hydride reactors is used for continuously developing power and refrigeration, following which the hydrogen gas, at reduced energy, is recycled to the reactors to recommence the HDH cycle. In order to chemically compress the hydrogen gas in the form of its hydride, a low-grade thermal source is utilized to supply heat to the several reactors.
In one aspect of the use of the HDH cycle as described in U.S. Pat. No. 3,943,719, the compressed and heated hydrogen gas which is released during the dehydriding phase of the HDH cycle is either passed directly to an expansion device, such as a turbine, or is first precooled via a heat exchanger before expansion. The cold exhaust from the power generating expansion device can be used in a heat exchanger to provide refrigeration prior to recharging the depressurized hydrogen to the HDH reactor bank.
The described methods of utilization of the high-pressure heated hydrogen gas, which is developed as a gaseous product discharged from the reactor bank in the course of carrying out the continuously operated HDH cycle, represent but a few of the uses which can be made of the hydrogen gas in its forms and energy states during the transition occurring between the time of dehydriding from the reactor bank in a pressurized state and the time the reactors are recharged to recommence the hydriding process.
Work carried on by Brookhaven National Laboratory for the United States Government has been proposed for a high efficiency power conversion cycle using hydrogen compressed by absorption to metal hydrides in a regenerative closed hydrogen Brayton cycle. In the cycle, hydrogen is thermochemically compressed using a low-temperature thermal energy source such as geothermal or solar energy, regeneratively heated, and then further heated by a high-temperature thermal source such as fossil or nuclear energy, and then expanded, reheated, and expanded again. The hydrogen is returned through the regenerators and then recompressed in the hydrides. Overall efficiency approaches 30 percent. However, high temperature energy efficiency, defined as the work output divided by the high temperature thermal input, approaches 90 percent.
Further work for the United States Government by the Naval Underwater Systems Center has proposed a heat pump cycle using hydrogen and hydrides. The system is comparable to conventional systems in that a mechanical compressor is used to compress the hydrogen, and absorption upon a base material supplies the heat effect of the heat pump cycle.
The Carnot cycle defines the limit of thermal efficiency not only for heat engine cycles and mechanical refrigeration cycles, but also for absorption cycles. The maximum efficiency for any cycle generating work from any thermal energy input is limited by the Carnot efficiency, which is defined as the net work produced, W.sub.net, divided by the heat input, Q.sub.H, and is equal to (Q.sub.H -Q.sub.Amb)/Q.sub.H =W.sub.net /Q.sub.H =(T.sub.H -T.sub.AMB)/T.sub.H. For mechanical refrigeration, the Carnot limit of thermal efficiency is defined as the heat absorbed by the cooling load, Q.sub.L, divided by the net work input, -W.sub.net, and is equal to QL/(Q.sub.Amb -Q.sub.L)=Q.sub.L /(-W.sub.net)=T.sub.L /(T.sub.Amb -T.sub.L). Q.sub.Amb is the available ambient heat sink.
An absorption system may be described as a combination heat engine-mechanical refrigeration system. The analogous and equivalent pairs of components between the heat engine and absorption systems are the condenser and absorber, the boiler pump and solution pump, and the boiler and generator. The expansion valve and expansion engine have analogous relations even through the expansion valve does not serve to remove work. The relations that are analogous between the mechanical refrigeration system and the absorption system are the evaporators, condensers, and expansion valves of both systems. The compressor of the mechanical refrigeration system does not have an analogous component in the absorption system since the working fluid gas of the absorption system has been compressed along with the absorbent in the solution pump. The maximum efficiency of an absorption cycle is thus defined with the work output of the expansion device in the heat engine system equal to the work input of the compressor of the mechanical refrigeration system, and is therefore Q.sub.L /Q.sub.H =(T.sub.L /T.sub.H) (T.sub.H -T.sub.Amb)/(T.sub.Amb -T.sub.L). The limit of efficiency defined here is for a two-component system. If the system had operated with a three-component system, then the ratio of Q.sub.L /Q.sub.H is on a different per-unit mass basis and the Carnot limit would not be the same as a two-component system. A three-component system might consist of ammonia and two organic or inorganic solvents. In a hydrogen absorption system, the hydrogen would be the primary working fluid, and any of several different classes of reversible hydriding materials could be utilized as the two or more absorbents. Therefore, the ratio Q.sub.l /Q.sub.H of an absorption system utilizing three or more components depends on the Carnot limit of thermal efficiency in a different way in that the limit is mass-dependent.
A heat pump system is essentially a mechanical refrigeration system with a different objective in view. The rejected energy in the refrigeration cycle becomes useful energy. The heat input is to be the evaporator from some ambient heat source. The efficiency is defined as the useful heat rejected. Q.sub.H, divided by the net work input, -W.sub.net, which is equal to Q.sub.H /-W.sub.net =Q.sub.H /Q.sub.H -Q.sub.Amb)=T.sub.H /(T.sub.H -T.sub.Amb). If an absorption system is again considered as a combination heat pump and heat engine system, with the heat engine operating with a heat source at ambient conditions and a heat sink at some lower temperature T.sub.L, the efficiency of the heat engine would be Q.sub.Amb '/W.sub.net =Q.sub.Amb '/(Q.sub.Amb '-Q.sub.L)=T.sub.Amb /(T.sub.Amb -T.sub.L). The combined absorption system efficiency can be defined again with the work output of the heat engine system equal to the work input to the heat pump system as Q.sub.H /Q.sub.Amb '=T.sub.H /T.sub.Amb) (T.sub. Amb -T.sub.L)/(T.sub.H -T.sub.Amb) where the Q.sub.Amb ' is only the heat input to the heat engine system. This defined efficiency of the absorption system is also massdependent for a system of three or more components.
A factor that is coming into more use recently is the concept of the energy utilization factor. This factor is defined as the desired energy transfer divided by the fuel input from the basic energy resource. Thus, typically, a natural gas furnace would have an E.U.F. of approximately 0.69, as 31 percent of the heat content of the natural gas is lost up the stack. For a heat pump, the E.U.F. is typically about 0.77, with the basic energy resource being the fossil fuel or nuclear fuel to an electric generating plant. The E.U.F. could be much higher for the heat pump if more of the energy, such as the rejected heat of the electric generating plant, were used. For example, if an electric generating plant received 1 Joule of thermal energy as a heat input and rejected 0.7 Joule, 0.3 Joule or energy would be produced as electrical energy. This 0.3 Joule of electricity used with a heat pump of 2.0 efficiency would produce 0.6 Joule of useful heat at the point of use. If the rejected heat of the electric generating plant could also be used, the E.U.F. would become 1.30. An absorption system using the combined systems of a heat engine and mechanical refrigeration analogy, and operating with a heat source of 1 Joule as a high-temperature input to the generator, could reject 2 Joules as a useful energy transfer such as heating a home, with one of the Joules coming from the heat engine condenser and the other Joule coming from the refrigerator condenser. The available refrigeration would also be 1 Joule, and the E.U.F. for such a system would be 3.0. This high efficiency is only possible if the efficiency is mass-dependent, as with a three-component system. Thus, effective utilization of the rejected heat of an absorption system of three or more components can mean a relatively high energy utilization factor.